Star
[2] A star's life begins with the gravitational collapse of a gaseous nebula of material largely comprising hydrogen, helium, and trace heavier elements.
Astronomers can determine stellar properties—including mass, age, metallicity (chemical composition), variability, distance, and motion through space—by carrying out observations of a star's apparent brightness, spectrum, and changes in its position in the sky over time.
They have been part of religious practices, divination rituals, mythology, used for celestial navigation and orientation, to mark the passage of seasons, and to define calendars.
By convention, astronomers grouped prominent stars into asterisms and constellations and used them to track the motions of the planets and the inferred position of the Sun.
[21] According to Josep Puig, the Andalusian astronomer Ibn Bajjah proposed that the Milky Way was made up of many stars that almost touched one another and appeared to be a continuous image due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars on 500 AH (1106/1107 AD) as evidence.
[22] Early European astronomers such as Tycho Brahe identified new stars in the night sky (later termed novae), suggesting that the heavens were not immutable.
Edmond Halley published the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed positions since the time of the ancient Greek astronomers Ptolemy and Hipparchus.
Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths.
[49] A number of private companies sell names of stars which are not recognized by the IAU, professional astronomers, or the amateur astronomy community.
[50] The British Library calls this an unregulated commercial enterprise,[51][52] and the New York City Department of Consumer and Worker Protection issued a violation against one such star-naming company for engaging in a deceptive trade practice.
The primordial binaries transfer some angular momentum by gravitational interactions during close encounters with other stars in young stellar clusters.
[86] In about 5 billion years, when the Sun enters the helium burning phase, it will expand to a maximum radius of roughly 1 astronomical unit (150 million kilometres), 250 times its present size, and lose 30% of its current mass.
The star then follows an evolutionary path called the asymptotic giant branch (AGB) that parallels the other described red-giant phase, but with a higher luminosity.
Because energy transport in an AGB star is primarily by convection, this ejected material is enriched with the fusion products dredged up from the core.
[92] These may instead evolve to a Wolf–Rayet star, characterised by spectra dominated by emission lines of elements heavier than hydrogen, which have reached the surface due to strong convection and intense mass loss, or from stripping of the outer layers.
[95] Some massive stars, particularly luminous blue variables, are very unstable to the extent that they violently shed their mass into space in events known as supernova impostors, becoming significantly brighter in the process.
If what remains after the outer atmosphere has been shed is less than roughly 1.4 M☉, it shrinks to a relatively tiny object about the size of Earth, known as a white dwarf.
The stars in an open or globular cluster all formed from the same giant molecular cloud, so all members normally have similar ages and compositions.
[129][130] When stars form in the present Milky Way galaxy, they are composed of about 71% hydrogen and 27% helium,[132] as measured by mass, with a small fraction of heavier elements.
Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements.
The magnetic field can act upon a star's stellar wind, functioning as a brake to gradually slow the rate of rotation with time.
The activity levels of slowly rotating stars tend to vary in a cyclical manner and can shut down altogether for periods of time.
[152] This represents an empirical value for the theoretical limit on the mass of forming stars due to increasing radiation pressure on the accreting gas cloud.
This generation of supermassive population III stars is likely to have existed in the very early universe (i.e., they are observed to have a high redshift), and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life.
[160][161] Smaller bodies called brown dwarfs, occupy a poorly defined grey area between stars and gas giants.
[188][189] The current stellar classification system originated in the early 20th century, when stars were classified from A to Q based on the strength of the hydrogen line.
[193] In addition, stars may be classified by the luminosity effects found in their spectral lines, which correspond to their spatial size and is determined by their surface gravity.
[194] There is additional nomenclature in the form of lower-case letters added to the end of the spectral type to indicate peculiar features of the spectrum.
For example, an "e" can indicate the presence of emission lines; "m" represents unusually strong levels of metals, and "var" can mean variations in the spectral type.
In a main sequence star such as the Sun, the lowest level of the atmosphere, just above the photosphere, is the thin chromosphere region, where spicules appear and stellar flares begin.
